MathDB
Fire on n islands

Source: 2023 Singapore MO Round 2 Senior Q3

June 24, 2023
combinatorics

Problem Statement

Let nn be a positive integer. There are nn islands with nāˆ’1n-1 bridges connecting them such that one can travel from any island to another. One afternoon, a fire breaks out in one of the islands. Every morning, it spreads to all neighbouring islands. (Two islands are neighbours if they are connected by a bridge.) To control the spread, one bridge is destroyed every night until the fire has nowhere to spread the next day. Let XX be the minimum possible number of bridges one has to destroy before the fire stops spreading. Find the maximum possible value of XX over all possible configurations of bridges and island where the fire starts at.